A hallmark characteristic of DCIS and invasive sporadic breast cancer (BC) is the high level of chromosomal instability (CIN) and aneuploidy in its tumor cells. However, CIN and aneuploidy have not been, until now, directly related to estrogen (E) action. The goal of this proposal is to determine the mechanism whereby E elicits CIN and aneuploidy in a unique, but relevant animal tumor model, the E-induced malignant tumors in the hamster kidney (HTK). SPECIFIC AIM 1. To determine the cell cycle components (cyclins E, D family, B1) and modulators (MDM2, DHFR) which exhibit sustained overexpression/amplification during E-induced HTK development. These genes/proteins have been shown to elicit CIN in in-vitro systems, and are downstream targets of c-myc/MYC overexpression/amplification mediated by E. SPECIFIC AIM 2. To determine whether the CIN detected during early E-induced HTK oncogenesis is caused by centrosome amplification resulting from the binding of cyclin.cdk complexes and other cell cycle modulators to purified HTK centrosomes and their proteins. These studies will provide evidence for a mechanism for initiating centrosome amplification, and hence CIN and aneuploidy in which these cyclin.cdk complexes and certain cell cycle modulators bind preferentially to specific centrosome proteins, or to different sites on the same centrosome protein. Another goal is to determine the earliest temporal appearance of centrosome amplification during early stages of HTK development. SPECIFIC AIM 3. To determine whether centrosome amplification (data performed in Sp. Aim 2) occurs before CIN and aneuploidy or as a consequence of them. This is a key issue in elucidating the mechanism of CIN and aneuploidy in HTKs. A major goal of this specific aim is to determine the earliest temporal appearance of CIN in early HTK loci, based on duration of E-treatment and HTK foci size (volume); using the same criteria for the earliest appearance of centrosome amplification. A second goal is to identify candidate genes that are involved in either CIN or growth advantage in the earliest stages of HTK development, employing the same techniques and DNA samples generated from LCMD and DOP-PCR of HTK loci to be subjected to cross-species mircoarray analysis.